The present invention relates to improved gel masses based on a reaction mixture of NCO prepolymers and selected compounds which contain groups reactive towards isocyanate groups, to a process for the production of these gel masses, and to the production of pressure-distributing elements or supports comprising these gel masses.
Gel masses may be used as pressure-distributing elements such as, for example, in cushions for wheelchairs, are principally described as being based on polyvinyl chloride, polyorganosiloxanes and polyurethanes, i.e. reaction products of polyols and polyisocyanates. Gel masses based on polyurethanes, such as those described for example in EP-A 057 838, have proved particularly advantageous for specifically adapting the properties of the gel mass to the particular intended end-use. Such polyurethane gel masses, which can be obtained from polyols of high molecular weight (i.e. hydroxyl numbers of 20 to 112) and polyisocyanates, have the disadvantage that the two reaction components have to be mixed in very different amounts. Therefore, in the mechanical production of these gel masses, the polyisocyanate component, which has to be used in comparatively small amounts, must be proportioned very accurately and with minimal fluctuations; otherwise, inhomogeneous gel masses with different consistencies are obtained.
By contrast, EP-A 511 570 describes polyurethane gel masses in which the components are used at a mixing ratio that allows a homogeneous blending of the components, especially when high-pressure machines are employed. The polyol component required consists of a mixture of polyols with hydroxyl numbers below 112, and polyols with hydroxyl numbers ranging from 112 to 600. The isocyanate index of the reaction mixture ranges from 15 to 59.81, and the product of isocyanate functionality and polyol functionality is at least 6.15. One disadvantage of these gel masses is that the mechanical properties can only be varied within narrow limits. Thus, for a low hardness such as, for example, a Shore 00<40, only a very limited recovery capacity can be achieved. Recovery capacity is understood by those of ordinary skill in the art as meaning the time taken by a shaped gel body to return to its initial height after a vertical compression of 50%. The very limited recovery capacity of these gel masses greatly restricts the possible fields of end-use.
EP-A 282 554 describes skin-friendly gel masses with inherent stickiness which can be used, for example, in skin plasters. The gel masses are obtained by reacting NCO prepolymers (prepared by reacting a polyisocyanate with a polyoxyalkylenediol monoalkyl ether) and a hydroxyl-containing compound such as, for example, water or a polyol. The polyols used can be, inter alia, are diols which are reacted in stoichiometric amounts with the NCO prepolymer. One disadvantage of these gel masses is that the mechanical properties can only be varied within narrow limits. Thus, if an attempt is made to obtain a low hardness such as, for example, a Shore 00<40, by reducing the isocyanate index, the resulting gel masses have a low recovery capacity or even dimensional instability.
Therefore, an object of the present invention was to provide gel masses which have low hardnesses, a good recovery capacity, and whose mechanical properties can be varied within wide limits. At the same time, it should be possible to blend the components which form the gel mass homogeneously when employing high-pressure machines, since this is the only way in which rapid, reactive gel systems can be processed.
Surprisingly, improved gel masses have been found which do not exhibit the disadvantages described above and which achieve the object with outstanding results.